dnn: make OpenCL DNN code optional

modules/dnn/src/layers/elementwise_layers.cpp

@@ -46,9 +46,12 @@
 #include "op_inf_engine.hpp"
 #include "opencv2/imgproc.hpp"
 #include <opencv2/dnn/shape_utils.hpp>
-#include "opencl_kernels_dnn.hpp"
 #include <iostream>
 
+#ifdef HAVE_OPENCL
+#include "opencl_kernels_dnn.hpp"
+#endif
+
 namespace cv
 {
 namespace dnn